This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. An absolute actinometer is being developed for quantitative characterization of singlet oxygen production upon one- and two-photon excitation for a specific system, e.g., nanosensitizer, DNA bases, etc. Photochemical reactions were carried out under pulsed laser at 532 nm and 355 nm in a cylindrical cell of 5 cm light path length for obtaining efficient photon absorption and 1O2 trapping using DPBF as a singlet oxygen acceptor in highly diluted solutions. A well-established Reincke's salt actinometer was used for the quantitative determination of incident photons at both 532 and 355 nm. FD were determined in chloroform solutions with satisfied accuracy and precision for some phthalocyanines. These compounds were used as standards for testing this method. Key factors that influence 1O2 production through the competitions for both triplet states and singlet oxygen were explored.